Routine monitoring of volcanic activity on Jupiter's moon
Io, now possible through advanced adaptive optics on the Keck II
telescope in Hawaii, has turned up the largest eruption to date on
Io's surface or in the solar system.

The eruption took place in February 2001, though image analysis was
only recently completed by a team of University of California,
Berkeley, astronomers. The group was co-led by postdoctoral research
associate Franck Marchis and Imke de Pater, professor of astronomy
and of earth and planetary science.

Their results are published in the November issue of the planetary
sciences journal Icarus.

"It is clear that this eruption is the most energetic ever seen, both
on Io and on Earth," Marchis said. "With the end of the Galileo
mission, ground-based telescopes equipped with adaptive optics
systems are the best tools for monitoring volcanic activity of Io. It
is clear that future monitoring of Io's volcanism lies in the hand of
terrestrial observers."

Adaptive optics employs a technique to remove the twinkle from stars,
by flexing segmented mirrors fast enough to stabilize and focus the
bouncing image created by turbulent air in the atmosphere.

Io, one of four large Jovian moons, is highly volcanic with
high-temperature eruptions similar to those common on Earth,
indicating a similar silicon-rich composition. The 2001 Io eruption
was very close to Surt, the site of a large eruption in 1979 that
took place between the Voyager 1 and Voyager 2 flybys.

"The Surt eruption appears to cover an area of 1,900 square
kilometers, which is larger than the city of Los Angeles and even
larger than the entire city of London," Marchis said. "The total
amount of energy being released by the eruption is amazingly high,
with the thermal output from this one eruption almost matching the
total amount of energy emitted by all of the rest of Io, other
volcanoes included."

The area covered by the Io lava is considerably larger than the
entire cone of one of Earth's most active volcanoes, Etna in Italy,
and far larger than Etna's most recent eruption in 1992.

"This eruption is truly massive," said Ashley Davies, PhD, a
scientist at NASA's Jet Propulsion Laboratory who aided in modeling
the eruption. "The observed energy indicates the presence of a
vigorous, high-temperature volcanic eruption. The kind of eruption to
produce this thermal signature has incandescent fire fountains of
molten lava which are kilometers high, propelled at great speed out
of the ground by expanding gases, accompanied by extensive lava flows
on the surface."

Io's volcanism has been monitored for the last eight years by the
Galileo spacecraft and now, with the advent of adaptive optics
systems, by Earth-bound astronomers. Ground-based observations with
an adaptive optics system, which produces very high-resolution
images, provide a competitive alternative to the limited temporal and
spatial coverage of Io by space missions, Marchis said. The spatial
resolution is 105 kilometers (66 miles) per pixel, comparable to many
infrared observations obtained by the Galileo Near Infrared Mapping
Spectrometer from orbit around Jupiter.

"We were lucky to detect the beginning of an outburst eruption," de
Pater said. "Thanks to the high-resolution capabilities of the
adaptive optics system, it was possible to pin-point the location of
the eruption, and the wavelength coverage allowed us to apply
constraints to the nature of the eruption."

With a specialized infrared camera, the Keck telescope captured
images of Io on two days, Feb. 20 and 22, at three different
wavelengths. On the first day, Io was mostly quiet, with visible
surface features such as dark calderas and relatively bright areas
rich in sulfur dioxide frost. Two days later, however, what seemed a
small hot spot on the surface had become a large bright eruption.

"We observed the same side of the satellite and were amazed to see a
very bright eruption that had suddenly appeared," Marchis said. The
UC Berkeley team quickly obtained data before Io entered the shadow
of Jupiter.

The data were analyzed using advanced image processing techniques and
a package called MISTRAL, developed by France's Office National
d'Etudes et de Recherche Aerospatiales (ONERA) MISTRAL yields clear
images of a quality comparable to observations taken above the
Earth's atmosphere. The data showed that the temperature of the
erupting lava was about 1,500 Kelvin, similar to that commonly seen
on Earth at locations such as the Hawaiian volcanoes.

The investigative team consists of Marchis and de Pater of UC
Berkeley, Davies of the Jet Propulsion Laboratory, UC Berkeley
graduate student Henry G. Roe, Thierry Fusco of ONERA, David Le
Mignant of the W. M. Keck Observatory, Pascal Descamps of the
Institut de Mecanique Celeste, Bruce A. Macintosh of Lawrence
Livermore National Laboratory and Renee Prange of the Institut
d'Astrophysique Spatiale.

This observational study of Io was supported by the France-Berkeley
Fund, the National Science Foundation and the Technology Center for
Adaptive Optics managed by UC Santa Cruz.

The observatory, located at the summit of Mauna Kea, provides
astronomers access to two 10-meter optical telescopes, the world's
largest. Each telescope features a revolutionary primary mirror
composed of 36 hexagonal segments that work in concert as a single
piece of reflective glass to provide unprecedented power and
precision. Both Keck telescopes are equipped with adaptive optics.

Funding for the telescopes and the Keck II adaptive optics system was
provided by the W.M. Keck Foundation. The observatory is operated by
the California Association for Research in Astronomy, a partnership
of the California Institute of Technology, the University of
California and the National Aeronautics and Space Administration
(NASA).
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NOTE: Images are available at
http://astron.berkeley.edu/~fmarchis/Science/Io/PressRelease/.

Franck Marchis can be reached in Hawaii via cellphone, (510) 599
0604, or after Nov. 15 at his UC Berkeley office, (510) 642 3958. His
email address is fmarchis@astro.berkeley.edu. Imke de Pater is at
(510) 642-1947 or imke@astro.berkeley.edu. Ashley Davies of the Jet
Propulsion Laboratory is at (818) 393-1775 or
Ashley.Davies@jpl.nasa.gov.